Differentials of Foot Drop

Question 1

Common Perineal Nerve Lesion

Answer 1

The sciatic nerve divides into the tibial and common peroneal nerves. Injury often occurs at the neck of the fibula

The most characteristic feature of a common peroneal nerve lesion is foot drop

Foot drop = Weakness of ankle dorsiflexion

Other features include:
weakness of foot dorsiflexion
weakness of foot eversion
weakness of extensor hallucis longus
sensory loss over the dorsum of the foot and the lower lateral part of the leg
wasting of the anterior tibial and peroneal muscles

Question 2

Foot Drop - Diagnosis: Example Question

Answer 2

A 70 year old male presents with 3 month history of left foot drop. He complains of having to lift his thighs higher than normal to accommodate this pathology. On examination, he has a high stepping gait. Power is normal in all movements except left ankle dorsiflexion (2/5) and eversion (2/5). Ankle inversion is intact (5/5), ankle jerks are present and plantars are downgoing. He reports reduced sensation on the dorsum of his foot. What is the diagnosis?

	L5 radiculopathy
	> Common peroneal palsy
	Functional neurology
	Sciatic nerve compression
	Lumbar plexopathy

The differential diagnosis of foot drop is a MRCP favourite. In this case, a number of features favour a common peroneal nerve palsy instead of a L5 radiculopathy: common peroneal nerve palsy patients normally have an intact ankle inversion and flexion of the big toe. Sensory loss is usually around the lateral aspect of the lower leg and the dorsum of the foot while L5 appears as a thin strip down the middle of the anterior lower limb, usually not affect the lateral lower leg. Ankle jerks are present in both, as the tibial nerve is branched from S1 and the sciatic nerve. There is little to suggest involvement of multiple lumbar levels in this scenario. The most common causes of common peroneal nerve palsy are trauma or compression at the fibula head, classically by tight plaster casts.

Question 3

Common Peroneal Nerve Lesion vs L5 Radiculopathy

Answer 3

Features favouring a common peroneal nerve palsy instead of a L5 radiculopathy:

Common peroneal nerve palsy patients normally have an intact ankle inversion and flexion of the big toe.

Sensory loss is usually around the lateral aspect of the lower leg and the dorsum of the foot while L5 appears as a thin strip down the middle of the anterior lower limb, usually not affect the lateral lower leg.

Ankle jerks are present in both, as the tibial nerve is branched from S1 and the sciatic nerve.

The most common causes of common peroneal nerve palsy are trauma or compression at the fibula head, classically by tight plaster casts whereas L5 radiculopathy would have to involve lumbar levels

Question 4

Common Peroneal Nerve Lesion vs L5 Radiculopathy

Answer 4

If weak ankle dorsiflexion and foot eversion, but strong ankle plantarflexion and foot inversion; numb dorsum of the foot; ankle jerk intact…. lesion is at the common peroneal nerve (usually compression at fibula neck) and NOT sciatic nerve or L5 nerve root.

L5 segment lesion: weak ankle dorsiflexion, foot eversion, AND foot inversion. Proceed to test hip internal rotation / abduction. Weak hip internal rotation / abduction indicates L5 radiculopathy.